I had to insert 15 seconds at the beginning of my sequence, so now all the markers are off by that amount of time.
I was wondering if it is possible to move all markers at once instead of doing it individually

In earlier versions of Premiere Pro, sequence markers were fixed on your timeline. When you deleted a part of a clip and closed the gap using ripple-delete, or inserted a clip using insert-edit, your markers wouldn't move with the clip.

Premiere Pro now lets you use Ripple Sequence Markers to let your markers ripple upstream or downstream when cutting or trimming in your timeline. For example, after an insert-edit, any sequence markers after the insertion point ripple downstream. Similarly, after a ripple-delete, any sequence markers after the last deleted frame ripple upstream.

This is extremely helpful, thank you.
To add on to this question -- can you copy markers along with clips when copy-pasting clips from one sequence to another? In other words, can you drag around the markers onto another timeline and/or can you permanently attach it to a clip?